1. Field of the Invention
The present invention relates to control means for automatic transmissions and more particularly to control means for automatic transmissions adapted to be used in motor vehicles such as automobiles.
2. Description of Prior Art
Conventional automatic transmissions generally include a hydraulic torque converter having an output shaft connected with a multiple-stage gear transmission mechanism such as a planetary gear mechanism. For selecting a desired one of the gear stages, the transmission usually includes a hydraulic system which are hydraulically operated or electromagnetically operated valves for appropriately selecting hydraulic circuits to thereby engage selected ones of friction devices such as clutches and brakes. The hydraulic system usually includes a manually operated shift valve which has a running range including D-range, 2-range, 1-range and R-range and a non-running range including N-range and P-range. Among the friction devices for selecting the gear stages of the gear transmission mechanism, there are a forward clutch which is adapted to be engaged in the D, 2 or 1-range and a reverse clutch which is adapted to be engaged in the R-range. Either one of these clutches is held in engagement even when the vehicle is not running as long as the shift valve is in the running range so that engine vibrations are transmitted through the transmission to the vehicle body. Such vibrations are particularly felt in idling operations of the engine.
In Japanese patent application No. 56-117742 filed on July 29, 1981 and disclosed for public inspection on Feb. 7, 1983 under the disclosure No. 58-21047, there is disclosed a transmission control in the pressure to the clutch is decreased, when the engine is in idling operation, below a level wherein the clutch can be engaged. With this proposal, it is possible to prevent the engine vibrations as produced in the idling operation from being transmitted to the vehicle body. However, the proposed control is primarily aimed to prevent a so-called creep phenomenon wherein the vehicle is slowly moved even under the idling operation of the engine by having the engine driving torque transmitted through the torque converter and the gear mechanism to the vehicle driving shaft. Therefore, with the control as proposed by this Japanese application, there is no means for transmitting the driving torque as long as the engine is in the idling operation. It should however be noted that there are some occasions wherein the creep phenomenon can be advantageously utilized. For example, in parking the vehicle in a very limited space or in driving in a traffic jam, it is very often necessary to move the vehicle at a very slow speed and the creep can advantageously be utilized for that purpose.